1. Field of the Invention
The present invention relates to surgical tools. More specifically, the present invention relates to thermally adjustable tools used in open and minimally invasive surgical procedures and interventional surgical and therapeutic procedures.
2. State of the Art
Surgery generally involves cutting, repairing and/or removing tissue or other materials. These applications are generally performed by cutting tissue, fusing tissue, or tissue destruction.
Current electrosurgery modalities used for cutting, coagulating, desiccating, ablating, or fulgurating tissue, have undesirable side effects and drawbacks.
Monopolar and bipolar electrosurgery modalities generally have disadvantages relating to “beyond the tip” effects. These effects are caused by passing alternating current through tissues in contact with conducting instruments or probes. One effect that is believed to be caused by both modalities is electrical muscle stimulation, which may interrupt surgical procedures and require administration of muscle relaxants.
Monopolar surgical instruments require electric current to pass through the patient. A return electrode is placed on the patient, often on the patient's thigh. Electricity is conducted from a “knife” electrode through the tissue and returns through the return electrode. Other forms of monopolar instruments exist, such as those which use the capacitive effect of the body to act as the return electrode or ground.
A low voltage high frequency waveform will incise, but has little hemostatic effect. A high voltage waveform will cause adjacent tissue hemostasis and coagulation. Therefore, when hemostasis is desirable, high voltage is used. The high voltage spark frequently has deeper tissue effects than the cut because the electricity must pass through the patient. The damage to the tissue extends away from the actual point of coagulation. Furthermore, there are complaints of return electrode burns. Yet, any reduction of voltage reduces the effectiveness of hemostasis. Further, the temperature of the spark or arc cannot be precisely controlled, which can lead to undesirable charring of target tissue.
Bipolar surgical instruments can produce tissue damage and problems similar to monopolar devices, such as sparking, charring, deeper tissue effects and electric current damage away from the application of energy with varying effects due to the differing electrical conductivity of tissue types, such as nerve, muscle, fat and bone, and into adjacent tissues of the patient. However, the current is more, but not completely, contained between the bipolar electrodes. These electrodes are also generally more expensive because there are at least two precision electrodes that must be fabricated instead of the one monopolar electrode.
Electrocautery resistive heating elements reduce the drawbacks associated with charring and deeper tissue damage caused by other electrosurgery methods. However, such devices often present other tradeoffs, such as the latency in controlling heating and cooling time, and effective power delivery. Many resistive heating elements have slow heating and cooling times, which makes it difficult for the surgeon to work through or around tissue without causing incidental damage.
Tissue destruction instruments generally heat tissue to a predetermined temperature for a period of time to kill, or ablate, the tissue. In some controlled heating of tissues, a laser is directed to an absorptive cap to reach and maintain a predetermined temperature for a predetermined amount of time. While this provides the benefits of thermal heating, it is expensive due to the complexity and expense of laser hardware.
In another tissue destruction procedure, a microwave antenna array is inserted into the tissue. These arrays are powered by instruments that cause microwave energy to enter and heat the tissue. While such devices are often effective at killing, or ablating, the desired tissue, they often cause deeper tissue effects than the desired area. Additionally the procedures can require expensive equipment.
Tissue destruction with resistively heated tools can produce unintended collateral tissue damage, in addition to having slow heating and cooling attributes.
Use of ferrite beads and alloy mixes in ceramics have been examined as alternatives. When excited by the magnetic field associated with high frequency current passing through a conductor, ferrite beads and alloy mixes in ceramics can reach high temperatures very quickly. However, one major problem with the use of these materials is that a large temperature differential can cause the material to fracture, especially when it comes into and out of contact with liquids. In other words, if a hot ferrite surgical instrument is quenched by a cooler pool of liquid, such as blood or other body fluids, the material's corresponding temperature drops rapidly and may cause the material to fracture. These fractures not only cause the tool to lose its effectiveness as a heat source, because the magnetic field is disrupted, but may require extraction of the material from the patient. Obviously, the need to extract small pieces of ferrite product from a patient is highly undesirable. Thus, there is a need for an improved thermal surgical tool.